1. Field of the Invention
The present invention relates to a CPAP (continuous positive airway pressure) device controlling pressure in a face mask. More particularly, the present invention relates to a device for supplying proper positive pressure by using a structure precisely measuring the breathing state of a patient.
2. Description of the Related Art
In general, when a person sleeps, all the muscles in the body of the person are relaxed. In addition to muscles of the trachea, muscles of the uvula or the throat are also relaxed, so the diameter of the trachea is reduced. Even if the diameter of the trachea is slightly reduced during sleeping, most persons feel no difficulty in breathing. However, some persons snore when breathing in due to the narrowed trachea or experience sleep apnea.
When the sleep apnea continues, the arterial oxygen saturation is excessively reduced, so circulatory disease such as hipertension, arrhythmia, ischemic heard disease, heart failure or cerebral palsy may easily occur. Further, the sleep apnea may cause insulin insufficiency, which is the main factor of diabetes, or may cause pulmonary disease/heart disease to become worse. In addition, the sleep apnea may cause sudden death during sleeping. According to statistics, a sleep apnea patient has a high incidence rate of hypertenstion (about twice as compared with that of ordinary people), arrhythmia (about twice as compared with that of ordinary people), coronary heart disease (about three times as compared with that of ordinary people), cerebrovascular (about four times as compared with that of ordinary people), and traffic accidents (about three times to about seven times as compared with that of ordinary people).
A CPAP therapy is representatively used for curing such a sleep apnea. According to the CPAP therapy, air having pressure is continuously supplied to the throat of a patient through a mask making closely contact with the nose of the patient during sleeping, so that the airway of the patient can be prevented from being obstructed. A positive airway pressure ventilator aids such a therapy. That is, the positive airway pressure ventilator supplies positive pressure air to a snoring or sleep apnea patient through a mask, who wears the mask on the nose during sleeping, thereby aiding the patient to easily breathe and thus curing the sleep apnea. This is the most secure and effective method of the non-surgical methods.
As shown in FIG. 6, a conventional CPAP device performing the functions as described above has a structure in which two pressure sensors are installed at the end portion of a flow supply catheter connected to a face mask while interposing a fluid resistor therebetween.
Referring to FIG. 6, since P2 reflects pressure in the flow supply catheter, an air stream generator is controlled to obtain a desired pressure value. At this time, an object, which must be substantially controlled, is pressure supplied to a patient, i.e. pressure Pm in the face mask. However, a scheme of controlling the air stream generator by using signals P1 and P2 has been employed on the assumption that the fluid resistor of the flow supply catheter is very low (Rc≈0) or Pm can be estimated from P2.
This is because it is inconvenient to install the pressure sensors, and the flow supply catheter and electric wire, which interconnect with the pressure sensors, around the face mask worn by a patient.
In order to estimate Pm from the pressure sensor P2, one fluid resistor R and one pressure sensor P1 must be additionally used. Accordingly, the air stream F supplied to the face mask is measured to estimate Pm. That is, since the fluid resistor is installed between P1 and P2, the air stream F has a value expressed by Equation 1 below.
                    F        =                                            P              1                        -                          P              2                                R                                    Equation        ⁢                                  ⁢        1            
When the fluid resistance of the flow supply catheter for air stream supply, which is connected to the face mask, is Rc, Equation 2 below is established.
                    F        =                                            P              2                        -                          P              m                                            R            c                                              Equation        ⁢                                  ⁢        2            
Equation 3 below is established from Equations 1 and 2.
                              P          m                =                                            (                              1                +                                                      R                    c                                    R                                            )                        ⁢                          P              2                                -                                                    R                c                            R                        ⁢                          P              1                                                          Equation        ⁢                                  ⁢        3            
Accordingly, Pm can be estimated from P1 and P2. At this time, if Rc has a value of 0, Pm becomes equal to P2. However, since the fluid resistance of the flow supply catheter is always larger than 0, P2>Pm. Thus, Pm cannot be estimated before P1 and P2 are measured.
The conventional CPAP device as described above has the following problems.
First, two pressure values must be measured to generate air stream such that Pm has a constant value (positive pressure) through Equation 3. Since P1 and P2 are essentially changed according to the generated air stream, the number (i.e. two) of control parameters is increased, causing the waste of circuit sources.
Second, providing that Rc has a value of 0 in Equation 3, P2 becomes equal to Pm and only P2 is controlled. However, since the fluid resistance of the flow supply catheter is always larger than 0, the control cannot be exactly performed.
Third, since the CPAP device is a medical instrument for preventing a patient from dying due to the sleep apnea during sleeping, it must be monitored whether the patient is spontaneously breathing. This can be achieved by detecting oscillation of pressure Pm in the face mask, which is caused by spontaneous breathing of the patient, on the basis of a target control value. In such a case, since P2 cooperates with Pm, a method of determining if the patient is spontaneously breathing by detecting the oscillation of P2 is used. However, since the flow supply catheter has a long length (>70 cm) and Rc≠0, P2 is less oscillated as compared with Pm. Thus, an error may occur in the breath detecting function.
Last, since two pressure sensors and one fluid resistor are used, the manufacturing cost is expensive.